The present invention relates to apparatus for spray coating a portion of a workpiece with a coating material and, more particularly, to such apparatus including a self-cleaning mask arrangement for preventing coating of other than the desired portion of the workpiece.
Various applications have arisen in which it is desired to spray coat only a portion of a workpiece. Masking tapes of special design, such as shown in U.S. Pat. No. 4,313,970, issued Feb. 2, 1982, to Jones et al, and in U.S. Pat. No. 4,033,803, issued Jul. 5, 1977, to Coder, have been developer to facilitate manual masking of a workpiece. In one application, it was desired to coat the lower portion of an automobile body with a layer of polyvinyl chloride material to protect the body from stone chips which may be thrown against it as the automobile is driven. To prevent extraneous coating of the upper portion of the vehicle body, the upper portion was covered with paper which was taped in place, with tape running along the periphery of the paper.
Several problems have been encountered with this manual masking approach. First, a phenomenon known as black lining has occurred. It is common for the coating material adjacent the edge of the material to take on a rather sharp edge as the masking tape is pulled away. The difficulty with this is that high solids paints will not adhere along the sharp edge, thus producing a stripe which is different in color than desired. Secondly, manual masking requires manual removal of the masking material. This manual demasking results in damage to an undesirably high percentage of coated workpieces. Finally, even if an acceptably coated workpiece is produced, the manual masking and demasking process is undesirably labor intensive and time consuming in nature.
Several attempts have been made to provide for limited, selective coating of a workpiece in a manner which does not require manually masking and unmasking the workpiece. Exemplary of such approaches are stencil arrangements shown U.S. Pat. Nos. 4,800,102, issued Jan. 24, 1989, to Takada, and 4,066,807, issued Jan. 3, 1978, to Craig. Both Takada and Craig recognize that it isd desirable to clean the stencils to eliminate build-up of the coating material and prevent the deterioration of the operation of the stencil which would otherwise necessarily result. Both Takada and Craig are limited to specialized, design coating applications, however.
Other coating applications have utilized spray deflectors to confine the area to be coated. U.S. Pat. No. 2,733,172, issued Jan. 31, 1956, to Brennan uses a pair of deflecting surfaces positioned to either side of a spray nozzle to confine the sprayed material to a moving web. These surfaces deflect and redirect the sprayed material onto the web. The apparatus disclosed in the Brennan patent is limited in its application to spray coating of materials that will not adhere to the deflecting surfaces, however.
It is seen, therefore, that there is a need for a spray coating process in which the workpiece need not be manually masked, but in which an acceptable coating operation is performed.